SENSORS: Quake detector preps for market

Portland, Ore.  As chairman of the physics department at Mercer University, Randall Peters is more accustomed to helping scientists than designing commercial products. But when he developed an instrument for NASA that detects the Earth's acceleration, he found he'd invented a marketable earthquake detector (see Jan. 17, 2005, page 36).

Peters' patented design is now slated to become a commercial product this year. Priced at under $500 for a bare-bones version, it will be the world's least expensive earthquake detector: The price of equally sensitive full-function seismic instruments is closer to $10,000.

Peters had set up his instrument to register the Earth's acceleration  but instead it registered an incredibly large earthquake and its aftershocks. By checking the exact timing of the quakes, Peters confirmed that his instrument  which he had built for less than $200 in parts  had detected the Dec. 26, 2004, earthquake in the Indian Ocean that caused the devastating tsunami in Southern Asia. Since then Peters has used his instrument from the comfort of his Macon, Ga., lab to monitor earthquakes worldwide.

Now Zoltech Corp. is building an instrument according Peters' design  called a volksmeter  "that we plan to bring to market, hopefully early next year [2006]," he said.

"We are finishing the prototype now," said Les LeZar, president of Zoltech (Van Nuys, Calif.). "As it stands, its case is just under 2 feet tall and houses an 18-inch pendulum."

Peters' design uses a novel means of varying the surface area of a capacitor. Rather than varying its gap as in standard capacitive sensors, Peters' design varies the capacitor's surface area. Because the capacitor's gap is constant, detection is not accompanied by a drop-off in sensitivity, as is the case with other capacitive sensors. Most of those become less sensitive when their gap widens. "I have a patent on several variations of what I call a symmetric differential capacitive sensor  what microelectromechanical-systems designers call 'fully differential,' " said Peters.

Since sensitivity and dynamic range don't have to be treated as a design trade-off  as in traditional capacitive sensors  Peters' design sets sensitivity by the constant size of the gap. By changing the surface area, it separately determines dynamic range.

In a nutshell, the dangling pendulum has a grounded Faraday shield on the end, hanging between the four plates of a symmetric differential capacitor wired in a diamond like a Wheatstone bridge, but with series diodes and parallel resistors to rectify its output into direct current. Parallel printed-circuit boards house the plates of four square capacitors.

The grounded Faraday shield initially covers all capacitors equally. When the Faraday shield is jiggled, it increases the surface area of two capacitors on opposite sides of the bridge, and decreases the other two opposing capacitors' area, thereby giving the bridge its differential sensitivity. A 50-MHz signal is then pumped across two opposing capacitor leads in the bridge, while the differential inputs to the sensing operational ampli-fier are wired to the other two opposing capacitor leads.

To make a commercial instrument that anybody could use, Zoltech designed autozeroing electronics for Peters' design. Autozeroing enables the device to work without calibration even if it's not completely level. When zeroed, all four capacitors begin with the same surface area, thereby giving a zero-rectified dc output from the bridge, but movements of just 25 nanometers are sensed by the differential op amp. This makes the seismic detector exceedingly sensitive without the need for sophisticated feedback networks.